1. Technical Field
The present invention relates to a method and an apparatus for outputting an audio signal, and more particularly, to a method and an apparatus for outputting an audio signal capable of removing noise generated when the audio signal is generated.
2. Related Art
As techniques of processing a digital signal have been developed, a method of processing an audio signal that employs a digital processing technique has also been developed. In a method of outputting a digital audio signal, a method of converting an analogue audio signal into a digital signal includes a pulse code modulation method. In the PCM method, the audio signal is digitalized, transmitted and accumulated.
A digital audio power amplifier that employs a digital pulse width modulation (PWM) method is used to output the pulse code modulated audio signal obtained in the PCM method with high reliability.
The high reliability of the digital audio power amplifier using the PWM method depends on a high accuracy DC source. Accordingly, any ripple caused by the power source influences on the PWM digital audio power amplifier. That is, the output audio signal may be damaged by the ripple of the power source.
Therefore, methods for solving the aforementioned problem have been suggested. First, a method of compensating for a damage of an audio signal due to a ripple of a power source is disclosed in U.S. Pat. No. 5,559,467, entitled “Digital, Pulse width modulation audio power amplifier with noise and ripple shaping” issued to Smedley. As disclosed in the U.S. Patent issued to Smedley, a divider outputs a value obtained by dividing an audio input signal x, which is over sampled and transmitted, by a source signal y. The source signal y is converted into a digital source signal by an analogue to digital (AD) converter and supplied, thereby generating the ripple.
That is, a relation z=x/y is satisfied. However, the PWM signal increases or decreases depending on a duty ratio with respect to the PCM signal that is the input signal x, and the duty ratio is non-zero during a PWM operation. However, the aforementioned equation suggested by Smedley does not consider the duty ratio of the PWM.
Accordingly, perfect compensation with respect to the PWM value cannot be obtained according to the U.S. Patent issued to Smedley.
There are practical problems caused by the source ripple in various fields in which the apparatus for outputting a digital audio signal is used. In a TDMA transmission method that is one of multiple access methods in mobile communication networks, a voltage drop due to a TDMA burst periodically occurs, thereby generating noise in an output audio signal.
To solve the problem, there is provided a technique. The technique is disclosed in Korean Patent Application No.2005-0003647 entitled “Method of removing noise from communication terminal” filed by Woongkil Choi. As disclosed in the Korean Patent Application, audio output noise is removed by applying an offset voltage during a burst period to reduce a battery voltage drop.
However, in the Korean Patent Application, only a source voltage drop is compensated, and however, the noise caused by the TDMA burst ripple is not prevented from being introduced into the audio output signal.